Transmissions called hydrostatic continuously variable transmissions (HST: Hydrostatic Transmission; hereafter called “HST ” as appropriate) are mounted in wheel loaders, wheel excavator, bulldozers, forklifts, and other work vehicles. As shown in FIG. 7, a closed-circuit HST includes a hydraulic pump 1 connected to the output shaft of an engine 4, a hydraulic motor 2 connected to a drive shaft 5a, and a closed circuit 3 linking the hydraulic pump 1 and the hydraulic motor 2.
The hydraulic pump 1 is driven by the engine 4, and pressurized oil is discharged from a port 1a at one side of the hydraulic pump 1, according to the travel direction of the vehicle. Pressurized oil discharged from the hydraulic pump 1 flows through an oil path 3a at one side into a port 2a at one side of the hydraulic motor 2. By this means, the hydraulic motor 2 is rotated, and the drive wheels 5 are driven. Pressurized oil flows out from a port 2b at another side of the hydraulic motor 2, passes through an oil path 3b at another side and is suctioned into a port 1b at another side of the hydraulic pump 1.
When a vehicle equipped with HST travels on a slope, the vehicle is accelerated by gravitational force, and the drive wheels 5 are rotated to generate a force equal to or stronger than the driving force of the hydraulic motor 2. As a result, the hydraulic motor 2 is caused to rotate and be driven by the driving wheels 5, and discharges high-pressure oil to the suction-side oil path 3b. Then, the high-pressure oil passes through the suction-side oil path 3b and is forced into the port 1b of the hydraulic pump 1, and the hydraulic pump causes the engine 4 to rotate and drive. Consequently there is a concern that the engine 4, hydraulic pump 1, and hydraulic motor 2 may rotate at a rotation rate exceeding the rated rotation rates (occurrence of overrun), causing oil film breakdown, cavitation, and other problems.
Further, while the vehicle is descending a slope at high speed, if the operator downshifts and the capacity of the hydraulic motor 2 increases, a larger quantity of high-pressure oil is discharged to the suction-side oil path 3b. Consequently, the high-pressure oil is forced through the suction-side oil path 3b into the port 1b of the hydraulic pump 1, and the hydraulic pump 1 causes the engine 4 to rotate and drive, so that overrun of the engine 4 and hydraulic pump 1 likely occurs. Hence, before downshifting, it is necessary to judge that the engine 4 and the hydraulic pump 1 do not exceed the rated rotation rates.
In Patent References 1 through 3 below, inventions are disclosed relating to vehicles equipped with HST, in which overrun is suppressed or downshifting is prohibited when the vehicle speed reaches or exceeds a prescribed value.
Patent Reference 1 discloses an invention in which the HST hydraulic motor capacity is adjusted according to the difference between the vehicle speed detected by a vehicle speed sensor and a preset limiting vehicle speed, to prevent overrun.
Patent Reference 2 discloses an invention in which, when the vehicle speed detected by a vehicle speed sensor reaches a prescribed value, the HST hydraulic motor capacity is increased to a capacity generating a prescribed braking force, to prevent overrun.
Patent Reference 3 discloses an invention in which. when the vehicle axle rotation rate (vehicle speed) detected by a vehicle axle rotation rate sensor connected to the HST hydraulic motor exceeds the allowable hydraulic motor rotation rate, the hydraulic motor rotation rate is reduced, to prevent overrun. This Patent Reference 3 also discloses an invention in which, when downshifting is performed, a judgment is made as to whether downshifting causes the HST hydraulic motor rotation rate to exceed the allowable rotation rate, and when it is judged that the allowable rotation rate is exceeded, downshifting is prohibited. but after the hydraulic motor rotation rate is reduced downshifting is executed, to prevent overrun.
Patent Reference 1: Japanese Patent Application Publication No. 2004-28229
Patent Reference 2: Japanese Patent Application Publication No. 2001-23503
Patent Reference 3: Japanese Patent Application Publication No. 11-62845